1. Field of the Invention
The present invention relates to an electromagnetic wave absorber of a broadband characteristic used for an electromagnetic wave anechoic room or the like.
2. Description of the Prior Art
An electromagnetic wave anechoic room is put to practical use widely as an examination room to measure an electromagnetic wave noise radiated by various electronic machines and to evaluate a tolerance of an electronic device interfered by an outside electromagnetic wave noise. And recently, there is a movement that the electromagnetic wave anechoic room is used for a place (CALTS=Calibration Test Site) to proofread an antenna for a radiation noise measurement.
Electromagnetic wave absorbers are installed in a ceiling and walls of these electromagnetic wave anechoic rooms for EMC (Electromagnetic Compatibility), therefore, a space is realized where electromagnetic wave reflections from the one except for a floor side (metal side) are very small.
A performance of an electromagnetic wave anechoic room for EMC is evaluated by measuring site attenuation. The site attenuation is an electromagnetic wave attenuation characteristic between transmission and reception antennas where it is measured in an established method in a predetermined measurement place. The site attenuation is measured in a frequency range of 30 MHz-1 GHz (or 18 GHz). Comparing ideal site attenuation (theoretical value) with a measured value of the site attenuation in an electromagnetic wave anechoic room, the electromagnetic wave anechoic room is high-performance as much as the difference is small between the theoretical value and the measured value. Usually, it is suitable as a measurement place of the radiation noise if the difference from the theoretical value is within the range of ±4 dB, but recently, there are many cases that ±3 dB is required, more case, high-performance of ±1 dB-±2 dB is required. It is because a radiation noise measurement of higher precision is provided as much as the difference from the theoretical value is small. If measurement precision in the electromagnetic wave anechoic room rises, electronic device makers can decrease a margin to a standard value when they measure the radiation noise of the products and confirm that the radiation noise is less than the standard value. As a result, there is an advantage to restrain a noise countermeasure cost.
On the other hand, because high precision is necessary when an electromagnetic wave anechoic room is used as a place to proofread an antenna, it requires high-performance of ±1 dB-±1.5 dB.
It is mostly said that an absorption characteristic of electromagnetic wave absorbers installed in a ceiling and walls of an electromagnetic wave anechoic room for EMC is required more than 20 dB with 30 MHz-18 GHz. The required characteristic depends on not only a performance of the electromagnetic wave anechoic room (difference between the theoretical value and the measured value of the site attenuation), but also a size of the electromagnetic wave anechoic room, a measurement distance and frequency and so on. Especially, a case of an electromagnetic wave anechoic room of 10 m method (the measurement distance is 10 m), the characteristic in low frequency of 30-100 MHz should be better than the characteristic in high frequency beyond 100 MHz. It results in terms of measurement of the site attenuation. In other words, it is because receiving electric field strength in the low frequency of 30-100 MHz is smaller than one in the high frequency beyond 100 MHz in case of a horizontal wave, so the reflected wave from the ceiling and the walls may influence the measured value, and the difference from the theoretical value grows large easily.
As an Electromagnetic wave absorber installed in the ceiling and the walls of the electromagnetic wave anechoic rooms for EMC, a complex type electromagnetic wave absorber is frequently used at present. The complex type electromagnetic wave absorber is, as shown in FIG. 9, a combination of a ferrite sintered compact 1 as an electromagnetic wave absorbent member consisting of magnetic loss material and a dielectric loss material 2 (This is also said an ohm loss factor, too.) as an electromagnetic wave absorbent member containing a conducting material.
The ferrite sintered compact absorbs electromagnetic waves by magnetic loss, and has an excellent characteristic in low frequency of about 30-400 MHz only with a thin thickness of several mm. On the other hand, The dielectric loss member is composed of a base material (low permittivity dielectric) such as foamed polystyrol or foamed polyurethane etc. containing a conducting material such as carbon or graphite or the like. The dielectric loss member absorbs electromagnetic waves by ohm loss, and has a better characteristic as much as frequency is high.
The complex type electromagnetic wave absorber is made to have the broadband characteristic by combining the ferrite sintered compact of excellent in low frequency characteristic and the dielectric loss member of excellent in high frequency characteristic. In comparison with usual wave absorber composed of only the dielectric loss member, the complex type electromagnetic wave absorber has a merit to make a length of the electromagnetic wave absorber less than half.
Usually, said dielectric loss member has a tapered shape such as a pyramid form or a wedge form or the like. The reason to provide the tapered shape is to receive and absorb electromagnetic waves efficiently with restraining reflection by making an impedance change gradually against incident electromagnetic waves from free space.
The dielectric loss member of 0.5-2 m in length is usually used, but there is a case that the member of 3 m and more in length is used according to the required performance of the electromagnetic wave anechoic room, because the dielectric loss member is higher performance as much as long one. So, for cost reduction with lightening and material reduction, shown in Japanese Patent Application Laid-Open No. 4-44300, an electromagnetic wave absorber of a hollow dielectric loss member is put to practical use. As a shape of the hollow dielectric loss member, there is a hollow pyramid structure shown in FIGS. 10A, 10B, and a hollow wedge structure shown in FIGS. 11A, 11B. In the FIGS. 10A, 10B and FIGS. 11A, 11B, numeral 1 is a ferrite sintered compact, 2 is a hollow dielectric loss member arranged to front of the ferrite sintered compact. Moreover, shown in Japanese Patent No. 3036252, and No. 3035110, they describe forms composed of a wedge shape structure by fitting two boards each other.
By the way, the hollow wedge structure and the wedge structure composed of fitting two boards each other have a problem that a difference in the characteristic is caused by a polarization plane of an arrival electromagnetic wave. A case of the wedge structure composed of fitting two boards each other, there is another problem in strength that each board cause sag or the like when a length of the boards is long.
On the other hand, a case of the hollow pyramid structure, there is no difference in the characteristic caused by the polarization plane of the arrival electromagnetic wave, and mechanical strength is strong. But, there is a problem that the absorber must be made long, because the low-frequency characteristic of 30-100 MHz was inferior in comparison with the hollow wedge structure.